CROSS-REFERENCE TO OTHER RELATED APPLICATIONS
This patent application is potentially related to the following commonly owned, copending applications:
1. "Method and System for Detecting Optical Faults Within the Optical Domain of a Fiber Communication Network," Ser. No. 08/580,391 by Shoa-Kai Liu, filed on Dec. 28, 1995, and incorporated herein by reference; PA1 2. "System and Method for Photonic Facility and Line Protection Switching," Ser. No. 08/575,663, now U.S. Pat. No. 5,731,887, by John A. Fee, filed on Dec. 22, 1995, and incorporated herein by reference; and PA1 3. "Method and System for Detecting Optical Faults in a Network Fiber Link," Ser. No. 08/582,845, Attorney Docket No. 15675.0520000, by John A. Fee, filed concurrently herewith, and incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to fiber optic telecommunication. More particularly, the present invention pertains to all-optical core network restoration.
2. Related Art
A communications network serves to transport information among a number of locations. The information to be transported is usually presented to the network in the form of time-domain electrical signals representing a combination of telephony, video, or computer data in a variety of formats. To transport such information, a typical communications network consists of various physical sites, and information conduits, called "links", that interconnect the sites. Each link serves to carry information from one site to another site. Each site or node may contain equipment for combining, separating, transforming, conditioning, and routing the information signals.
The traffic of even a single link represents a formidable volume of vital information flow--equivalent to tens of thousands of phone calls. Sudden failure of a link can cause a significant loss in revenues for a network owner and loss of commerce and other benefits for the network subscribers. Consequentially, restoration techniques have been devised to circumvent failure and restore normal traffic flow as quickly as possible.
Optical fibers are increasingly relied upon for carrying vital communications traffic. Fiber trunks extend between nodes (i.e. cities) forming networks extending across cities, states, nations, and continents. Fiber cables whether laid out above ground, underground, or underwater are subject to damage. For example, lightning, backhoes, fires, train derailment, trawler nets, and shark attacks have been reported to have severed or impaired optical fiber connectivity. See, Grover, Wayne PhD., "Distributed Restoration of the Transport Network," Network Management into the 21st Century, Chapter 11, IEEE Press (1994), p 337.
Fiber optic cables carry far greater amounts of digital data than conventional electrical cables. A single fiber operating at 1.2 Gb/s and packeting data according to a standard SONET OC-24 protocol, carries the equivalent of over 16,000 voice circuits. Future fiber demands project over 130,000 callers per fiber. In an OC-192 wavelength-division multiplexing scheme, eight wavelengths are available to carry a data volume equivalent to over one million voice calls in a single fiber| Moreover, dozens of fibers may be included in a single cable. The impact of a cable cut, or even a single optical fiber or nodal failure, can be widespread. Fiber network survivability has become critical to implementing an effective fiber communication.
To avoid susceptibility to a single point failure, sophisticated networks include redundant fibers, called protect or back-up fibers, to link nodes. Optical line switches have also been proposed to switch the flow of optical data traffic from the working fiber to the protect fiber in the event of a detected failure along the working fiber. See, e.g., the diverse protect fiber network architecture described by Wu et al., "Strategies and Technologies for Planning a Cost-Effective Survivable Fiber Network Architecture Using Optical Switches," CH2655-9 I.E.E.E., pp. 749-55, (1989) (incorporated by reference herein).
Optical switches, however, introduce significant loss. The actual loss varies by switch technology. An optical signal passing through Integrated Lithium-Niobate switches, such as the 4.times.4 switch made by NEC, loses approximately six decibels. Furthermore, the magnitude of the loss suffered by a system, generally increases with port count. A 128.times.128 NEC switch introduces a 50 db. loss| Such loss reduces the long-distance range of the fiber link.
What is needed is an all-optical restoration system and method whereby data can be re-routed through optical switching in the event of fiber failure or other system error without introducing line loss.